The present invention relates to high pressure bellows assemblies and more particularly to an improved restraining means for limiting the expansion of tandem bellows segments in high pressure installations.
The bellows assemblies to which the present invention relates are those employed to maintain a high pressure seal between relatively movable parts. For example, in a high pressure valve which includes a housing having inlet and outlet ports connected by a passageway, a plug fixed to a movable stem is movable into and out of engagement with a valve seat in the passageway to alternately block and allow passage of pressurized fluid from the inlet to the outlet. The valve stem extends through a pressure chamber in the housing and outwardly thereof for manual manipulation. A packing seal surrounds the stem to prevent loss of pressure from the chamber past the valve stem.
Because of the high pressures involved and the sudden surge of pressure into the chamber occurring when the valve is first opened, it has become customary to employ a bellows seal connected at opposite ends to the housing and the movable valve stem to confine the pressurized fluid to the valve chamber. Any pressurized fluid leaking past the bellows seal is easily handled by the stem packing.
One of the problems which has arisen in these bellows seals, particularly where the bellows has considerable length, is non-uniform expansion and contraction of the bellows convolutions which occurs during valve stem movement. The end convolutions are the first to expand when the valve stem is moved in a direction expanding the bellows and are consistently exposed to greater fluid pressure (and more rapid wear) than the intermediate convolutions.
Also, high pressure in the system can cause some of the bellows convolutions to remain stacked (contracted) so that when the valve stem is moved in a direction requiring bellows expansion, other convolutions are caused to overextend. Again, fluid pressure and wear concentrate at the overextended convolutions and result in premature bellows failure.
One approach to solving these problems is to replace the single bellows length with shorter, individual, tandemly arranged bellows segments which are connected by floating collars. To insure that each bellows segment expands properly and to limit the maximum expansion for each bellows segment, the valve housing and stem are both formed with stop surfaces which, during bellows expansion, cooperate with corresponding ones of the floating collars. The shorter length of the tandemly arranged bellows segments and the insurance that each segment expands properly and that no convolutions overextend greatly enhance the life expectancy of the bellows assembly.
However, this arrangement requires that both the valve housing and valve stem be specially designed with stop surfaces which are positioned to cooperate with the floating collars in the bellows assembly. This involves several expensive machining operations of both the valve housing and valve stem and requires careful matching of bellows assemblies to valve housings and stems.
It has also been proposed to use bellows assemblies provided with headed elongated rods which are freely (floatingly) disposed within tandem bellows segments. Here, the headed rods abut the floating collars and limit the expansion of each bellows segment. The disposition of the rods internally of the bellows segments makes this construction difficult to build and to service and repair. Furthermore, the floating or loose mounting of the rods relative to the floating collars results in noisy operation and a device which is susceptible to failure in use.